It is known to connect together mating inner and outer components using a tolerance ring. For example, a tolerance ring may be sandwiched between a shaft that is located in a corresponding bore formed in a housing, or it may act as a force limiter to permit torque to be transmitted between the shaft and the housing. The use of a tolerance ring accommodates minor variations in the diameter of the inner and outer components without substantially affecting their interconnection.
Typically, a tolerance ring comprises a band of resilient material, e.g. a metal such as spring steel, the ends of which are brought towards one another to form a ring. A strip of projections extends radially from the ring either outwardly or inwardly towards the centre of the ring. The projections can be formations, possibly regular formations, such as corrugations, ridges, waves or fingers. The band thus comprises an unformed region from which the projections extend, e.g. in a radial direction. There may be one or more rows of projections.
In use, the tolerance ring is located between the components, e.g. in the annular space between the shaft and bore in the housing, such that the projections are compressed between the inner and outer components. Typically, all of the projections extend either outwardly or inwardly so that one of the inner and outer component abuts projections and the other abuts the unformed region. Each projection acts as a spring and exerts a radial force against the components, thereby providing an interference fit between them. Rotation of the inner or outer component will produce similar rotation in the other component as torque is transmitted by the ring. Likewise, linear movement of either component will produce similar linear movement in the outer component as linear force is transmitted by the ring.
If forces (rotational or linear) are applied to one or both of the inner and outer components such that the resultant force between the components is above a threshold value, the inner and outer components can move relative to one another, i.e. the tolerance ring permits them to slip.
Typically tolerance rings comprise a strip of resilient material that is curved to allow the easy formation of a ring, e.g. by overlapping the ends of the strip.
During assembly of apparatus with an interference fit between components, a tolerance ring is typically held stationary with respect to a first (inner or outer) component whilst a second component is moved into mating engagement with the first component, thereby contacting and compressing the projections of the tolerance ring to provide the interference fit. The amount of force required to assemble the apparatus may depend on the stiffness of the projections and the degree of compression required. Likewise, the load transmitted by the tolerance ring in its final position and hence the amount of retention force provided or torque that can be transmitted may also depend on the size of the compression force and the stiffness and/or configuration of the projections.
One example of the use of a tolerance ring is in a hard disk drive (HDD) pivot mount, where the tolerance ring provides axial retention between a rotatable pivot shaft and an arm mounted thereon. In conventional pivot mounts, the tolerance ring provides an interference fit between the arm and a bearing mounted on the shaft. Typically the bearing comprises two pairs of races which are axially separated from each other by a spacer. Since the components in pivot mounts are very small and sensitive, the bearing is often protected by a surrounding sleeve (a “sleeved pivot”). The sleeve often has the spacer machined on its inner surface. In such arrangements the tolerance ring is sandwiched between the sleeve and the arm. Whilst sleeved pivots are less prone to damage and therefore are less likely to adversely affect hard disk drive performance, the precise machining required to form the spacer on the inner surface of the sleeve and the desire to use less material in the manufacture of pivot mounts has led to the introduction of sleeveless pivots.
In sleeveless pivots, the outer race of each part of races is exposed, and the spacer comprises an annular band located axially (“floating”) between them. The spacer is held in place by an axial pre-loading force exerted on the bearing. In such arrangements the tolerance ring is located between the outer races of the bearing and the arm.
The coupling between mating components may exhibit resonant behavior, i.e. where external vibrations are amplified in the coupling. The resonant frequency or frequencies of an assembly are important in determining the operation of that assembly. For example, in hard disk drive pivot mounts accurate data writing cannot take place when resonance occurs, so it is important to know the frequency of resonance. The resonant frequency may depend on amount of compression that takes place during installation.